If you are expendable expect to be exploited. Its why so many programmers go out of their way to become needed - if you are the only one who properly understands critical code in your business, you have leverage. If they can replace you with a week of onboarding you are going to be treated like dirt most places.

Sometimes that means not going out of your way to explain how you implemented a feature. If your company is going to treat you like an expendable money sink treat them like a hostile actor too.